Devices for predicting cardiovascular abnormalities are known, and are often integrated into (or connected to) an implanted cardiac pacemaker or cardioverter/defibrillator. It is desirable to design these prediction devices such that they predict a dysfunction of the cardiovascular system well in advance of the dysfunction, and with high specificity.
Implants for predicting decompensation of the heart of a patient are known, such as from US Patent Appl'n. Publ'n. 2008/0157980. Some of these implants perform prediction based on the detection of a parameter having a value dependent on a rate of fluid in the lungs of the patient. It is known that the buildup of water in pulmonary tissue is an indicator of imminent decompensation. A known approach is to quantify the rate of fluid in the pulmonary tissue by capturing transthoracic electrical impedance. The disadvantage here is that the electrical conductivity of the blood, which is subject to natural fluctuations, has considerable influence on the impedance measurement, so that prediction devices based on this approach have a comparatively low specificity.
Another way to determine the rate of fluid in pulmonary tissue is to determine the sound velocity of an acoustic signal passing through the lungs of the patient. It is known that the sound velocity is dependent on the density of the medium through which the sound wave travels. Unfortunately, this method suffers from the disadvantage that the sound velocity is dependent not only on the rate of fluid in the pulmonary tissue, but also highly dependent on the breathing cycle, i.e., the fluctuation of the air volume in the lungs. This principle has further disadvantages, which will be described below.
A prediction device which operates via the acoustic method is described in Patent Appl'n. Publ'n. US 2002/0123674 A1. The sound velocity is determined by way of travel time measurement, and the reference describes the travel time of an ultrasonic pulse being in the range of 10 to 100 μs. In order to be able to detect fluid buildup in the pulmonary tissue and an associated change in the travel time, the signal processor in the described prediction device must have a resolution of approximately 100 ns, which is difficult to achieve without significant complexity. Additionally, a change in the travel time of this magnitude could be caused by a change in the distance between the transmitter and receiver of approximately 0.1 mm. As a result, the exact distance between the transmitter and receiver must be known in order to be able to provide reliable information about the change in sound velocity. Very small changes in the distances, such as those caused by a natural change in the size of the lungs or by variably deep breathing processes, can result in travel time changes, which can be mistaken for a change in the sound velocity and thereby possibly fluid buildup in the pulmonary tissue. Thus, a prediction device of this type tends to exhibit low specificity, particularly after an extended idle time during which component distance changes tend to occur. This disadvantage, as well as the need for a complex signal processing system, decreases the suitability of the acoustic method for implementation in implantable medical devices.